U.S. Pat. Nos. 4,859,858 and 4,859,859, both entitled GAS ANALYZERS, were issued to Knodle et al. on Aug. 22, 1989. Both patents disclose state-of-the art apparatus for outputting a signal indicative of the concentration of a designated gas in a sample being monitored by the apparatus.
The gas analyzers disclosed in the '858 and '859 patents are of the non-dispersive type. They operate on the premise that the concentration of a designated gas can be measured by: (1) passing a beam of infrared radiation through the gas, and (2) then ascertaining the attenuated level of the energy in a narrow band absorbable by the designated gas. This is done with a detector capable of generating a concentration proportional electrical output signal.
One important application of the invention at the present time is in capnometers for monitoring the level of carbon dioxide in the breath of a medical patient. This is typically done during a surgical procedure as an indication to the anesthesiologist of the patient's condition, for example. As the patient's wellbeing, and even his life, is at stake, it is of paramount importance that the carbon dioxide concentration be measured with great accuracy.
In a typical instrument or system employing non-dispersive infrared radiation to measure gas concentration, including those disclosed in the '858 and '859 patents, the infrared radiation is emitted from a source and focused by a mirror on the gases being analyzed. After passing through the body of gases, the beam of infrared radiation passes through a filter. That filter absorbs all of the radiation except for that in a narrow band centered on a frequency which is absorbed by the gas of concern. This narrow band of radiation is transmitted to a detector which is capable of producing an electrical output signal proportional in magnitude to the magnitude of the infrared radiation impinging upon it. Thus, the radiation in the band passed by the filter is attenuated to an extent which is proportional to the concentration of the designated gas. The strength of the signal generated by the detector is consequently inversely proportional to the concentration of the designated gas and can be inverted to provide a signal indicative of that concentration.
While a non-dispersive analyzer must be tailored to the specific gas of interest, it is typically small, relatively cheap, and rugged enough to be used in medical and other demanding environments.
Most non-dispersive infrared gas analyzers use a ratioing scheme to eliminate errors attributable to drifts in the infrared source and other parts of the system and transmission losses. Two methods are common.
1. An optical chopper is used with a single detector. The chopper contains a reference cell or filter, and the detector signal alternates between that reference cell and the gas to be measured. A ratio is taken of these two signals.
2. Two detectors are located next to each other, and each is illuminated by one-half of the infrared beam. A ratio is taken of the two detector outputs. The reference channel is presumed to be responsive to any changes in the detected energy that are not due to the absorption of the designated gas, and the changes are presumed to be the same in both the reference and data channels.
A major drawback of the optical chopper technique is that it requires a device with moving parts to implement it. Such devices tend to be expensive, bulky, and fragile and to require frequent calibration.
Another difficulty, common to both schemes, is that the ratioed signals are different in time in the first case and different in space in the second. These differences can produce a false signal from the detector if there are time variations in the first case or spatial variations in the second case.
Also, any motion in the system can cause time and spatial variations in the infrared radiation beam. At best, this can require frequent recalibration. Recalibration is time consuming and expensive and takes the unit or system out of operation which may be unacceptable -- for example, a major surgical procedure cannot be interrupted simply to recalibrate an instrument.
Dispersive infrared analysis is also utilized to measure the concentration of a designated gas in a stream or other sample being analyzed. In this approach, a broad band of energy is transmitted through the gas, then through a dispersive element, typically a prism or a diffraction grating. The dispersive element spreads out the energy according to wavelength. The intensity of the energy will vary across that space depending on the absorption characteristics of the gas under analysis.
A detector scans through a large range of wavelengths, thereby recording intensity (i.e., absorption of the gas) as a function of wavelength. A manual or electronic examination of this recording will identify the gas or gases that may h=present. The detector can also be fixed in space, thus recording the intensity of a particular wavelength (and gas) of interest.
The dispersive type of gas analyzer is especially useful where an unknown gas may be present because a very large range of wavelengths can be covered. It is also useful where there are several gases present.
The dispersive analyzer is flexible and can be very accurate, but it is expensive and bulky. It is most often found in a laboratory, and it is not at all suitable for applications such as those medical applications in which the entire emitter/detector system may have to be suspended in the plumbing between a patient and a mechanical ventilator, for example.